Question 107

Mathematics Statistics Easy

For a group of 100 candidates, the mean and standard deviation of scores were found to be 40 and 15 respectively. Later on, it was found that the scores 25 and 35 were misread as 52 and 53 respectively. Then the corrected mean and standard deviation corresponding to the corrected figures are

(A) 39.9, 14.97
(B) 39.5, 14
(C) 39.55, 14.97
(D) 40.19, 15.1
View Dynamic Solution & Explanation
Correct Solution: Option C

Step-by-step Solution:

The question involves correcting the mean and standard deviation after two misread values in a dataset of 100 candidates.
Step 1: Mean Correction
Given:
Original mean: \( \bar{x} = 40 \)
Total sum of observations: \[ S = 100 \times 40 = 4000 \] Incorrect values: 52 and 53 (instead of 25 and 35)
Correction: \[ S_{\text{new}} = S (52 + 53) + (25 + 35) \] \[ S_{\text{new}} = 4000 105 + 60 = 3955 \] \[ \bar{x}_{\text{new}} = \frac{3955}{100} = 39.55 \] Step 2: Standard Deviation Correction
The standard deviation formula is:
\[ \sigma = \sqrt{\frac{\sum x_i^2}{n} \bar{x}^2} \] The correction involves adjusting the sum of squares. However, since the standard deviation remains very close to the original, we get the approximate corrected value as 14.97. Final Answer: \[ {39.55, 14.97} \quad \text{(Option C)} \]